Optical coupling between a single mode fiber (SMF) and a silicon planar waveguide is one of the key issues in photonic integrated circuits fabricated in silicon-on-insulator (SOI) technology due to mode mismatch arising from the significantly higher refractive index of the silicon planar waveguide compared to that of the SMF. Various types of mode size converters (i.e., optical couplers) have been proposed to address this mismatch issue such as, for example, taper structures and grating couplers.
The grating coupler provides surface coupling and can have large bandwidth up to several tens of nanometers. Such an optical coupler can be appropriate for functional wafer-scale testing of photonic integrated circuits during the fabrication process because there is no need to cleave or dice devices, which is required using edge-coupling (e.g., taper) structures. Furthermore, the grating couplers are not only useful for coupling light into an SMF but are also suitable for integrating surface mountable active devices, such as a photodiode and a vertical-cavity surface-emitting laser. However, existing grating couplers are strongly polarization-dependent due to their geometry. Several approaches have been proposed to overcome polarization-dependent limitations such as, for example, polarization splitting grating couplers (called two-dimensional or 2D gratings) with polarization diversity circuits, and the polarization-independent grating couplers for dual polarizations into a single silicon waveguide (called one-dimensional or 1D grating). The 2D grating coupler structure requires polarization diversity schemes such as complex polarization rotators or two identical functional devices without rotators, one for each polarization state. The schemes make the circuit more complicated and efficient polarization-independent fiber-to-chip coupling is still challenging. Therefore, simpler 1D grating couplers are preferable.
The 1D grating coupler approach has been proposed for the waveguide on a SOI wafer such as using thick silicon waveguides, using double etched gratings, or using double surface corrugation. The refractive index contrast (Δ) of a silicon waveguide, defined as (ncore2−nclad2)/2ncore2 is ˜48%, where core refers to thin glass center of the optical fiber where light travels, and clad or cladding refers to the outer optical material surrounding the core that reflects light back into the core. For single mode operation with this Δ, the typical size of silicon waveguide is approximately 400 nm by 220 nm in width and thickness, respectively. Waveguide thickness greater than 220 nm may not provide single mode operation even though the grating coupler formed with the thick-waveguide can have better coupling efficiency and design flexibility. Designs based on double etched grating and double surface corrugation are undesirable due to the complicated fabrication process.